Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize its surface. The charged portion of the photoconductive surface is exposed to a light image from a scanning laser beam or an LED source that corresponds to an original document being reproduced. The effect of the light on the charged surface produces an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises a mixture of magnetic carrier granules and toner particles that adhere triboelectrically to the latent image. A single-component developer material is typically comprised of toner particles without carrier particles. Toner particles are attracted to the latent image, forming a toner powder image on the latent image of the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet to form the hard copy image.
One common type of development system uses one or more donor rolls to convey toner to the latent image on the photoconductive member. A donor roll is loaded with toner either from a two-component mixture of toner and carrier particles or from a single-component supply of toner. The toner is charged either from its triboelectric interaction with carrier beads or from suitable charging devices such as frictional or biased blades or from other charging devices. As the donor roll rotates it carries toner from the loading zone to the latent image on the photoconductive member. There, suitable electric fields can be applied with a combination of DC and AC biases to the donor roll to cause the toner to develop to the latent image. Additional electrodes, such as those used in the Hybrid Scavengeless Development (HSD) technology may also be employed to excite the toner into a cloud from which it can be harvested more easily by the latent image. The process of conveying toner, sometimes called developer, to the latent image on the photoreceptor is known as “development.”
A problem with donor roll developer systems is a defect known as ghosting or reload, which appears as a lightened ghost image of a previously developed image in a halftone or solid on a print. Reload defect occurs when insufficient toner has been loaded onto the donor roll within one revolution of the donor roll after an image has been printed. The donor roll retains the memory of the image, and a ghost image shows up, if another image is printed at that time.
One way of improving the ability of the toner supply to provide an adequate amount of toner to reduce or prevent ghost images is to increase the peripheral speed of the magnetic brush or roll that transfers toner from the supply reservoir to the donor roll. As the relative difference in the speed of the magnetic and donor rolls increases so do the collisions of the carrier or toner granules as well. The toner particles also impinge on the blade mounted proximate to the magnetic brush to regulate, or trim, the height of the magnetic brush so that a controlled amount of toner is transported to the developer roll. The collisions of the toner with the carrier and the trim blade tend to smooth the surface of the toner particles and cause the particles to exhibit increased adhesion. This increased adhesion causes the toner particles to adhere more strongly to the donor roll, and less toner is transferred to the photoreceptor to develop the latent image at a given development voltage. The reduction in the developability of the toner particles is sometimes known as toner abuse.
The stability of the toner may be monitored by maintaining a historical log of the development voltage necessary to provide adequate toner density. As the development system loses the ability to develop toner on the latent image, the absolute value of the development voltage is increased. As the development voltage absolute value approaches the maximum of the development system, corrective action is required to restore the ability of the development system to develop the toner.
What is needed is a way of reducing the abuse of the toner without causing the reload or ghosting defect.